The use of sodium and various sodium compositions, such as NaK, in nuclear facilities necessitates a highly reliable and sensitive liquid level measuring device to monitor the level of the sodium and sodium compositions. For instance, there are requirements for liquid level probes for monitoring the liquid metal levels in vessels, tanks and closed systems in fast flux nuclear reactor test facilities.
Inductive bifilar wound coil probes for sensitive liquid levels are well known in the prior art. The construction and operation of such a device is described in copending Patent Application Ser. No. 587,792, entitled "Liquid Metal Level Sensor With Increased Sensitivity", filed June 16, 1975 now U.S. Pat. No. 3,996,801 by the assignee of the present invention. The liquid metal level mutual inductance type probe typically consists of an elongated former or support mandrel which is bifilar wound with twin lengths of insulated wire to form two closely coupled coils, one being a primary and the other a secondary, extending over the active length of the liquid metal level probe. The combination of the closely wound coil turns of primary and secondary windings is enclosed in a closed end protective tubular housing which isolates the coil turns from direct contact with liquid metal present in a liquid metal containment into which the closed end tubular housing is immersed. The primary winding is AC excited, and due to the closed coupling with the secondary winding, a signal is induced into the secondary winding. The amplitude of the induced signal is controlled by the level of the liquid metal surrounding the closed end tubular protective housing, thus forming an AC analog of the liquid metal level.
Typically, the insulated wire used to form the coil turns of the primary and secondary windings is coaxial cable in which the conductive outer sheath is brazed to the supporting metal mandrel with the center conductor of adjacent coaxial cables functioning as the primary and secondary windings. The brazing of the outer sheath establishes the desired bulk effect mode of operation of the mutual inductance type liquid level probe.
While the above-identified conventional mutual inductance type liquid level probe, which is further described and illustrated in U.S. Pat. Nos. 3,818,759 and 3,834,231, generally exhibits a linear analog output signal over a substantial center portion of the active length of the probe defined by the coil windings, these conventional mutual inductance liquid metal probes exhibit substantial nonlinearity at the end portions of the probe, wherein the end portion is typically defined as having a minimum length, as measured from the last active coil, approximating the effective diameter of the probe. While this non-linearity does not pose a significant problem in applications where the mechanical length of the probe can be of sufficient length to minimize the end effect, numerous closed loop and tank applications limit the overall length of the probe such that significant linearity of the output signal developed over the entire length of the active portion of the probe is essential.